C++ has an absurd amount of UB to the point where there's an entire thought exercise devoted to how malicious a compliant compiler can be (Hell++.) There are things that are allowed and even common in C but UB in C++ (union type punning,) and things that people reasonably can assume would work but are UB anyway (signed integer overflow.) Then you have weird edge cases like assigning the return value of a two argument std::max involving temporaries to a reference. There are so many UB foot guns, no reasonable developer can be expected to keep track of them all.